The present invention relates to laser tuning.
In the optical communication industry there is a need for testing optical components and amplifiers with lasers that can be tuned in wavelength continuously without mode hopping. To perform these tests Littman cavities can be used as external cavities to allow single-mode tuning of the laser. The geometry of these cavities is known, see e.g.: Liu and Littman, xe2x80x9cNovel geometry for single-mode scanning of tunable lasersxe2x80x9d, Optical Society of America, 1981, which article is incorporated herein by reference. The advantage of the Littman cavity is that it is possible to tune the wavelength and the optical length of the cavity at the same time by changing only one parameter of the geometry, i.e. the tuning element.
Examples of tunable lasers, in particular based on the Littman geometry, can be found e.g. in U.S. Pat. No. 5,867,512, DE-A-19509922, Wenz H. et al: xe2x80x9cContinuously Tunable Diode Laserxe2x80x9d in xe2x80x98Laser und Optoelekronikxe2x80x99 (Fachverlag GmbH, Stuttgart, DE, Vol.28 No.1, p.58-62, Feb. 1, 1996, XP000775842, ISSN: 0722-9003), Wandt D. et al: xe2x80x9cContinuously Tunable External-Cavity Diode Laser with a Double-Grating Arrangementxe2x80x9d (Optics Letter, Optical Society of America, Washington, US, vol.22, no.6, Mar. 15, 1997, pages 390-392, XP000690335, ISSN: 0146-9592), DE-A19832750, EP-A-938171, JP-A-05 267768, or U.S. Pat. No. 5,319,668.
The Littman geometry, however, is extremely sensible to deviations of the real geometry with respect to the perfect Littman configuration. This does impose severe requirements on the rotation mount for the tuning element of the Littman cavity. Smallest errors in the positioning of the pivot axis of the tuning element reduce the mode hop free tuning range of the cavity heavily. This requires costly precision when manufacturing and maintaining such tunable lasers.
Therefore, it is an object of the invention to provide improved tuning of a laser. The object is solved by the independent claims.
An advantage of the present invention is the provision of a tunable laser that autonomously and easily compensates for deviations, e.g. a shift of the real position of the pivot axis of the tuning element with respect to the theoretical perfect position of the pivot axis. This compensation is sufficient to provide continuous single-mode tuning within a predetermined tuning range of the tuning element. The compensation is done by moving the dispersion element, preferably along a predetermined path, corresponding with the rotation of the tuning element. Therefore, method and an apparatus of the present invention for tuning of lasers avoid the aforementioned problems of the prior art and provide a tunable laser with a wide mode hop free tuning range without heavy duties to the precision when manufacturing and maintaining such laser.
In a preferred embodiment of the present invention the moving of the dispersion element is done simultaneously with the rotation of the tuning element. This achieves an online correction, so that always the correct position of the dispersion element for the full feedback of the tuning element is guaranteed.
In another preferred embodiment of the invention the correction is done by moving the dispersion element by rotating it by a predetermined rotating angle about a predetermined rotating axis. This way of correction can be easily implemented in the inventive apparatus, e.g. by using a piezo-electric driven rotation stage that can precisely move the respective tuning element of the laser.
In yet another preferred embodiment of the invention the correction is done by a dispersion element which is a diffraction grating and in which the rotating axis is at least not perpendicular, more preferred parallel, to the axes of the rules of the grating. This positioning serves for maximum efficiency of the inventive method and apparatus.
In another preferred example of the invention the method further comprises steps for at least approximately evaluating a function which determines the rotating angle of the dispersion element for generating mode or wavelength hop free rotating of the tuning element within a predetermined tuning range of the tuning element per rotation angle of the tuning element. This evaluation is done by the following steps: step a: substantially detecting mode or wavelength hops during rotation of the tuning element, step b: rotating the tuning element about a predetermined angle until at least one mode or wavelength hop substantially has occurred, step c: rotating the dispersion element about an arbitrary angle, step d: rotating back the tuning element about the predetermined angle of step a, and repeating steps a to d with increasing or decreasing rotating angle of step c until substantially no mode or wavelength hops during rotation of the tuning element are detected in step b, and using the overall rotating angle of step c per rotating angle of step b to evaluate an approximation of the function which determines the rotating angle of the dispersion element per rotating angle of the tuning element. This can be done fast and easy so that a quick adjustment of the apparatus for full feedback of the tuning element is achieved.
After performing the above described determination it is preferred to move the dispersion element according to the approximation function before or while rotating the tuning element.
Other preferred embodiments are shown by the dependent claims.
It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.